The present invention relates to a bridge, in particular a bridge of a very large span, of the type comprising a deck, at least two towers and a certain number of cables or stays connecting the tops of the towers to the deck in order to support the latter.
Hitherto, bridges of a very large span (greater than 1000 m) have been constructed with suspended decks. The most simple form of these known structures comprises one or more main suspension cables, tensioned between two towers, above which they are deflected in order to be anchored at the end of the side spans in powerful anchor blocks. The deck carrying the traffic (road, railway, fluid conduits, etc.) is suspended from the suspension cables by suspenders which are generally approximately vertical and regularly spaced out along the length of the structure.
With the materials currently available (framework steel and steel for suspension cables), the maximum clear span of such structures is greater than 3000 m; however, the cost of the suspension cables and of the anchor blocks increases extremely quickly with the span. Furthermore, the vertical deformations of the deck and the variations in the longitudinal inclination of the latter under the passage of the moving loads (lorries or trains) soon become critical. In order to limit the bending and rotations to acceptable values, structures must be built which are highly surbased and in which the height of the towers above the deck is 1/10 to 1/9 of the clear span, in other words the distance between two successive towers. This limitation further increases the weight and the cost of the suspension cables and of their anchor blocks.
In order to overcome these disadvantages, for approximately the last thirty years engineers have turned to cable-stayed bridges. The deck is suspended from multiple stays distributed uniformly over its length, generally in an approximately symmetrical manner on either side of each tower The vertical loads of the deck are divided into a tension sustained by the stays and a compression sustained by the deck. The tensions of the stays are generally selected in such a way that the reaction force imposed on the tower is vertical, with the result that the compressions in the deck are balanced on either side of the tower. The height of the towers can be selected to be much larger than for suspension bridges 1/5 to 1/4.5 of the clear span, with the result that the cost of the stays is reduced whilst increasing the rigidity of the structure. Lastly, the anchor blocks are no longer necessary, which represents a considerable saving in the overall cost of the structure.
On the other hand, the deck is now subjected to substantial compressive forces which must be taken into account in the calculations. For a deck sustaining a total load (permanent loads +moving loads) w per unit length, and assuming that all the stays are anchored to the top of the tower, the axial compressive force N in the deck varies parabolically from zero (at the crown of the central span or at the end of the side span) to a maximum value at right angles to the tower equal to N=wa.sup.2 /2h, a being the distance from the tower to the crown of the central span or to the end of the side span, and h being the height of the tower above the deck. It can be seen that a doubling of the span, all other things being equal, results in a quadrupling of the compressive load. (For the sake of simplification, the weight of the stays has been ignored in this expression). With the properties of the current materials, the limit span of a cable-stayed bridge lies between 1000 and 1500 m; it is determined by the exhaustion of the compressive strength of the deck under the effect of the axial force (plus, of course, the various thermal effects and the bending moments created by the passage of the moving loads).
In its field of application, the cable-stayed bridge is more rigid than a suspension bridge and substantially more economical. This intrinsic advantage is confirmed by the fact that in the last twenty years, 10 times more cable-stayed bridges have been constructed than suspension bridges in the range of clear spans from 200 to 800 m.
In order to widen the field of application of cable-stayed bridges beyond their current limit span, the idea was mooted of combining the two systems of staying and suspension. In its most simple form, this combination consists in constructing, from each tower, two traditional cable-stayed decks over a first length on either side of each tower. The central part of the main gap, over a second length on either side of the crown, is then suspended from a cable which is itself anchored in external blocks by vertical suspenders. Such a solution is described, in particular, in "Connaissance des ouvrages d'art No 3-4, 1988-89: Darius Amir-Mazaheri A 3000-meter bridge - an advance in the study thereof", pages 68-71.
More complex, so-called "net and lattice" solutions have also been proposed, see in particular "Cable supported Bridges, Concept and Design", by Niels GIMSING, published by John Wiley and Sons, pages 176-183. In the structure proposed by this author, it is possible to distinguish deck parts supported in the traditional manner by stays anchored at each of their ends at points situated on either side of towers these stays being deflected at the top of the corresponding tower; these deck parts being followed, towards the middle of the central span, by cable-stayed parts in which the stays, at their other end, are anchored in an anchor block situated beyond the side span. The bridge furthermore comprises a short central part which is supported, via vertical or inclined suspenders, by a suspension cable which joins the same anchor blocks to the ends of the bridge. There may also be a partial overlapping between this "suspended" part and the adjacent cable-stayed part. The horizontal forces resulting from the action of the weight of the deck on the stays and the suspenders are balanced by a compressive force in the cable-stayed parts of the deck, a tensile force in the central part of the deck, and a tensile force in the suspension cable. It is possible, for example, to calculate the lengths of the parts of the bridge in such a way that these three forces are equal.
These mixed solutions have as yet not got beyond the designer stage and no structure of this type has been made. This is probably because such designs attempt to combine in one and the same structure two fundamentally different techniques: stays on the one hand and suspension cables and suspenders on the other hand. Not only are the structural behaviours different, but the materials and the technology for the construction are also very different.
It has also been proposed, Swiss Patent 447,247, to support the central part of the span exclusively by stays which are anchored, on the one hand, in anchor blocks situated beyond the deck and are deflected in the upper part of the towers, and, on the other hand, towards the ends of the central part. This central part is then subjected, between the stays which are deflected by one tower and those which are deflected by the other, to a considerable tensile stress, which limits the dimensions which it is possible to give this central part.
The object of the present invention is to eliminate such difficulties and thus to bring multiple-cable-stayed bridges into the range of span previously reserved for suspension bridges.